lib/mpi: bug fixes and cleanup

Former v2 can be found here:

http://lkml.kernel.org/g/[email protected]

This v3 series incorporates a fix to the pointer arithmetic issue in v2's [8/14] ("lib/mpi: mpi_read_buffer(): fix buffer overflow") spotted by Tadeusz Struk.

The rest, that is [1-7,9-14/14], go unchanged and have got a

already.

Applicable to linux-next-20160322.

Changes to v2:
- [8/14] ("lib/mpi: mpi_read_buffer(): fix buffer overflow")
+ Fix the pointer arithmetic issue found by Tadeusz Struk

Changes to v1:
- [1-8/14]

    former [1-8/8], unchanged.

- [9-14/14]
Added in v2. Fixes to mpi_read_raw_from_sgl().

Nicolai Stange (14):
lib/mpi: mpi_write_sgl(): fix skipping of leading zero limbs lib/mpi: mpi_write_sgl(): fix style issue with lzero decrement lib/mpi: mpi_write_sgl(): purge redundant pointer arithmetic lib/mpi: mpi_write_sgl(): fix out-of-bounds stack access lib/mpi: mpi_write_sgl(): replace open coded endian conversion lib/mpi: mpi_read_buffer(): optimize skipping of leading zero limbs lib/mpi: mpi_read_buffer(): replace open coded endian conversion lib/mpi: mpi_read_buffer(): fix buffer overflow
lib/mpi: mpi_read_raw_from_sgl(): replace len argument by nbytes lib/mpi: mpi_read_raw_from_sgl(): don't include leading zero SGEs in
nbytes
lib/mpi: mpi_read_raw_from_sgl(): purge redundant clearing of nbits lib/mpi: mpi_read_raw_from_sgl(): fix nbits calculation lib/mpi: mpi_read_raw_from_sgl(): sanitize meaning of indices lib/mpi: mpi_read_raw_from_sgl(): fix out-of-bounds buffer access

 lib/mpi/mpicoder.c | 122 +++++++++++++++++++----------------------------------
 1 file changed, 43 insertions(+), 79 deletions(-)

2.7.4

Currently, if the number of leading zeros is greater than fits into a complete limb, mpi_write_sgl() skips them by iterating over them limb-wise.

However, it fails to adjust its internal leading zeros tracking variable, lzeros, accordingly: it does a

  p -= sizeof(alimb);
  continue;

which should really have been a

  lzeros -= sizeof(alimb);
  continue;

Since lzeros never decreases if its initial value >= sizeof(alimb), nothing gets copied by mpi_write_sgl() in that case.

Instead of skipping the high order zero limbs within the loop as shown above, fix the issue by adjusting the copying loop's bounds.

---
 lib/mpi/mpicoder.c | 21 +++++++++------------
 1 file changed, 9 insertions(+), 12 deletions(-)

index eb15e7d..6bb52be 100644
@@ -380,7 +380,9 @@ int mpi_write_to_sgl(MPI a, struct scatterlist *sgl, unsigned *nbytes,
 	buf_len = sgl->length;
 	p2 = sg_virt(sgl);
 
-	for (i = a->nlimbs - 1; i >= 0; i--) {
+	for (i = a->nlimbs - 1 - lzeros / BYTES_PER_MPI_LIMB,
+			lzeros %= BYTES_PER_MPI_LIMB;
+		i >= 0; i--) {
 		alimb = a->d[i];
 		p = (u8 *)&alimb2;
 #if BYTES_PER_MPI_LIMB == 4

@@ -401,17 +403,12 @@ int mpi_write_to_sgl(MPI a, struct scatterlist *sgl, unsigned *nbytes,
 #error please implement for this limb size.
 #endif
 		if (lzeros > 0) {
-			if (lzeros >= sizeof(alimb)) {
-				p -= sizeof(alimb);
-				continue;
-			} else {
-				mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
-				mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
-							+ lzeros;
-				*limb1 = *limb2;
-				p -= lzeros;
-				y = lzeros;
-			}
+			mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
+			mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
+				+ lzeros;
+			*limb1 = *limb2;
+			p -= lzeros;
+			y = lzeros;
 			lzeros -= sizeof(alimb);
 		}
 

2.7.4

Within the copying loop in mpi_write_sgl(), we have

  if (lzeros > 0) {
    ...
    lzeros -= sizeof(alimb);
  }

However, at this point, lzeros < sizeof(alimb) holds. Make this fact explicit by rewriting the above to

  if (lzeros) {
    ...
    lzeros = 0;
  }

---
 lib/mpi/mpicoder.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 6bb52be..d8b372b 100644
@@ -402,14 +402,14 @@ int mpi_write_to_sgl(MPI a, struct scatterlist *sgl, unsigned *nbytes,
 #else
 #error please implement for this limb size.
 #endif
-		if (lzeros > 0) {
+		if (lzeros) {
 			mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
 			mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
 				+ lzeros;
 			*limb1 = *limb2;
 			p -= lzeros;
 			y = lzeros;
-			lzeros -= sizeof(alimb);
+			lzeros = 0;
 		}
 
 		p = p - (sizeof(alimb) - y);

2.7.4

Within the copying loop in mpi_write_sgl(), we have

  if (lzeros) {
    ...
    p -= lzeros;
    y = lzeros;
  }
  p = p - (sizeof(alimb) - y);

If lzeros == 0, then y == 0, too. Thus, lzeros gets subtracted and added back again to p.

Purge this redundancy.

---
 lib/mpi/mpicoder.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

index d8b372b..78ec4e1 100644
@@ -407,12 +407,11 @@ int mpi_write_to_sgl(MPI a, struct scatterlist *sgl, unsigned *nbytes,
 			mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
 				+ lzeros;
 			*limb1 = *limb2;
-			p -= lzeros;
 			y = lzeros;
 			lzeros = 0;
 		}
 
-		p = p - (sizeof(alimb) - y);
+		p = p - sizeof(alimb);
 
 		for (x = 0; x < sizeof(alimb) - y; x++) {
 			if (!buf_len) {

2.7.4

Within the copying loop in mpi_write_sgl(), we have

  if (lzeros) {
    mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
    mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
                               + lzeros;
    *limb1 = *limb2;
    ...
  }

where p points past the end of alimb2 which lives on the stack and contains the current limb in BE order.

The purpose of the above is to shift the non-zero bytes of alimb2 to its beginning in memory, i.e. to skip its leading zero bytes.

However, limb2 points somewhere into the middle of alimb2 and thus, reading *limb2 pulls in lzero bytes from somewhere.

Indeed, KASAN splats:

  BUG: KASAN: stack-out-of-bounds in mpi_write_to_sgl+0x4e3/0x6f0
                                      at addr ffff8800cb04f601
  Read of size 8 by task systemd-udevd/391
  page:ffffea00032c13c0 count:0 mapcount:0 mapping:   (null) index:0x0
  flags: 0x3fff8000000000()
  page dumped because: kasan: bad access detected
  CPU: 3 PID: 391 Comm: systemd-udevd Tainted: G  B  L
                                              4.5.0-next-20160316+ #12
  [...]
  Call Trace:
   [<ffffffff8194889e>] dump_stack+0xdc/0x15e
   [<ffffffff819487c2>] ? _atomic_dec_and_lock+0xa2/0xa2
   [<ffffffff814892b5>] ? __dump_page+0x185/0x330
   [<ffffffff8150ffd6>] kasan_report_error+0x5e6/0x8b0
   [<ffffffff814724cd>] ? kzfree+0x2d/0x40
   [<ffffffff819c5bce>] ? mpi_free_limb_space+0xe/0x20
   [<ffffffff819c469e>] ? mpi_powm+0x37e/0x16f0
   [<ffffffff815109f1>] kasan_report+0x71/0xa0
   [<ffffffff819c0353>] ? mpi_write_to_sgl+0x4e3/0x6f0
   [<ffffffff8150ed34>] __asan_load8+0x64/0x70
   [<ffffffff819c0353>] mpi_write_to_sgl+0x4e3/0x6f0
   [<ffffffff819bfe70>] ? mpi_set_buffer+0x620/0x620
   [<ffffffff819c0e6f>] ? mpi_cmp+0xbf/0x180
   [<ffffffff8186e282>] rsa_verify+0x202/0x260

What's more, since lzeros can be anything from 1 to sizeof(mpi_limb_t)-1, the above will cause unaligned accesses which is bad on non-x86 archs.

Fix the issue, by preparing the starting point p for the upcoming copy operation instead of shifting the source memory, i.e. alimb2.

---
 lib/mpi/mpicoder.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

index 78ec4e1..b05d390 100644
@@ -403,15 +403,11 @@ int mpi_write_to_sgl(MPI a, struct scatterlist *sgl, unsigned *nbytes,
 #error please implement for this limb size.
 #endif
 		if (lzeros) {
-			mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
-			mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
-				+ lzeros;
-			*limb1 = *limb2;
 			y = lzeros;
 			lzeros = 0;
 		}
 
-		p = p - sizeof(alimb);
+		p = p - sizeof(alimb) + y;
 
 		for (x = 0; x < sizeof(alimb) - y; x++) {
 			if (!buf_len) {

2.7.4

Currently, the endian conversion from CPU order to BE is open coded in mpi_write_sgl().

Replace this by the centrally provided cpu_to_be*() macros.

---
 lib/mpi/mpicoder.c | 27 +++++++++++----------------
 1 file changed, 11 insertions(+), 16 deletions(-)

index b05d390..623439e 100644
@@ -20,6 +20,7 @@
 
 #include <linux/bitops.h>
 #include <linux/count_zeros.h>
+#include <linux/byteorder/generic.h>
 #include "mpi-internal.h"
 
 #define MAX_EXTERN_MPI_BITS 16384

@@ -359,7 +360,13 @@ int mpi_write_to_sgl(MPI a, struct scatterlist *sgl, unsigned *nbytes,
 		     int *sign)
 {
 	u8 *p, *p2;
-	mpi_limb_t alimb, alimb2;
+#if BYTES_PER_MPI_LIMB == 4
+	__be32 alimb;
+#elif BYTES_PER_MPI_LIMB == 8
+	__be64 alimb;
+#else
+#error please implement for this limb size.
+#endif
 	unsigned int n = mpi_get_size(a);
 	int i, x, y = 0, lzeros, buf_len;
 

@@ -383,22 +390,10 @@ int mpi_write_to_sgl(MPI a, struct scatterlist *sgl, unsigned *nbytes,
 	for (i = a->nlimbs - 1 - lzeros / BYTES_PER_MPI_LIMB,
 			lzeros %= BYTES_PER_MPI_LIMB;
 		i >= 0; i--) {
-		alimb = a->d[i];
-		p = (u8 *)&alimb2;
 #if BYTES_PER_MPI_LIMB == 4
-		*p++ = alimb >> 24;
-		*p++ = alimb >> 16;
-		*p++ = alimb >> 8;
-		*p++ = alimb;
+		alimb = cpu_to_be32(a->d[i]);
 #elif BYTES_PER_MPI_LIMB == 8
-		*p++ = alimb >> 56;
-		*p++ = alimb >> 48;
-		*p++ = alimb >> 40;
-		*p++ = alimb >> 32;
-		*p++ = alimb >> 24;
-		*p++ = alimb >> 16;
-		*p++ = alimb >> 8;
-		*p++ = alimb;
+		alimb = cpu_to_be64(a->d[i]);
 #else
 #error please implement for this limb size.
 #endif

@@ -407,7 +402,7 @@ int mpi_write_to_sgl(MPI a, struct scatterlist *sgl, unsigned *nbytes,
 			lzeros = 0;
 		}
 
-		p = p - sizeof(alimb) + y;
+		p = (u8 *)&alimb + y;
 
 		for (x = 0; x < sizeof(alimb) - y; x++) {
 			if (!buf_len) {

2.7.4

Currently, if the number of leading zeros is greater than fits into a complete limb, mpi_read_buffer() skips them by iterating over them limb-wise.

Instead of skipping the high order zero limbs within the loop as shown above, adjust the copying loop's bounds.

---
 lib/mpi/mpicoder.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

index 623439e..2fd8d41 100644
@@ -184,7 +184,9 @@ int mpi_read_buffer(MPI a, uint8_t *buf, unsigned buf_len, unsigned *nbytes,
 	p = buf;
 	*nbytes = n - lzeros;
 
-	for (i = a->nlimbs - 1; i >= 0; i--) {
+	for (i = a->nlimbs - 1 - lzeros / BYTES_PER_MPI_LIMB,
+			lzeros %= BYTES_PER_MPI_LIMB;
+		i >= 0; i--) {
 		alimb = a->d[i];
 #if BYTES_PER_MPI_LIMB == 4
 		*p++ = alimb >> 24;

@@ -205,15 +207,11 @@ int mpi_read_buffer(MPI a, uint8_t *buf, unsigned buf_len, unsigned *nbytes,
 #endif
 
 		if (lzeros > 0) {
-			if (lzeros >= sizeof(alimb)) {
-				p -= sizeof(alimb);
-			} else {
-				mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
-				mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
-							+ lzeros;
-				*limb1 = *limb2;
-				p -= lzeros;
-			}
+			mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
+			mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
+				+ lzeros;
+			*limb1 = *limb2;
+			p -= lzeros;
 			lzeros -= sizeof(alimb);
 		}
 	}

2.7.4

Currently, the endian conversion from CPU order to BE is open coded in mpi_read_buffer().

Replace this by the centrally provided cpu_to_be*() macros. Copy from the temporary storage on stack to the destination buffer by means of memcpy().

---
 lib/mpi/mpicoder.c | 27 ++++++++++++---------------
 1 file changed, 12 insertions(+), 15 deletions(-)

index 2fd8d41..a999ee1 100644
@@ -21,6 +21,7 @@
 #include <linux/bitops.h>
 #include <linux/count_zeros.h>
 #include <linux/byteorder/generic.h>
+#include <linux/string.h>
 #include "mpi-internal.h"
 
 #define MAX_EXTERN_MPI_BITS 16384

@@ -164,7 +165,13 @@ int mpi_read_buffer(MPI a, uint8_t *buf, unsigned buf_len, unsigned *nbytes,
 		    int *sign)
 {
 	uint8_t *p;
-	mpi_limb_t alimb;
+#if BYTES_PER_MPI_LIMB == 4
+	__be32 alimb;
+#elif BYTES_PER_MPI_LIMB == 8
+	__be64 alimb;
+#else
+#error please implement for this limb size.
+#endif
 	unsigned int n = mpi_get_size(a);
 	int i, lzeros;
 

@@ -187,25 +194,15 @@ int mpi_read_buffer(MPI a, uint8_t *buf, unsigned buf_len, unsigned *nbytes,
 	for (i = a->nlimbs - 1 - lzeros / BYTES_PER_MPI_LIMB,
 			lzeros %= BYTES_PER_MPI_LIMB;
 		i >= 0; i--) {
-		alimb = a->d[i];
 #if BYTES_PER_MPI_LIMB == 4
-		*p++ = alimb >> 24;
-		*p++ = alimb >> 16;
-		*p++ = alimb >> 8;
-		*p++ = alimb;
+		alimb = cpu_to_be32(a->d[i]);
 #elif BYTES_PER_MPI_LIMB == 8
-		*p++ = alimb >> 56;
-		*p++ = alimb >> 48;
-		*p++ = alimb >> 40;
-		*p++ = alimb >> 32;
-		*p++ = alimb >> 24;
-		*p++ = alimb >> 16;
-		*p++ = alimb >> 8;
-		*p++ = alimb;
+		alimb = cpu_to_be64(a->d[i]);
 #else
 #error please implement for this limb size.
 #endif
-
+		memcpy(p, &alimb, BYTES_PER_MPI_LIMB);
+		p += BYTES_PER_MPI_LIMB;
 		if (lzeros > 0) {
 			mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
 			mpi_limb_t *limb2 = (void *)p - sizeof(alimb)

2.7.4

Currently, mpi_read_buffer() writes full limbs to the output buffer and moves memory around to purge leading zero limbs afterwards.

However, with

commit 9cbe21d8f89d ("lib/mpi: only require buffers as big as needed for
the integer")

the caller is only required to provide a buffer large enough to hold the result without the leading zeros.

This might result in a buffer overflow for small MP numbers with leading zeros.

Fix this by coping the result to its final destination within the output buffer and not copying the leading zeros at all.

the integer")

---
 lib/mpi/mpicoder.c | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

index a999ee1..d995a4c 100644
@@ -201,16 +201,9 @@ int mpi_read_buffer(MPI a, uint8_t *buf, unsigned buf_len, unsigned *nbytes,
 #else
 #error please implement for this limb size.
 #endif
-		memcpy(p, &alimb, BYTES_PER_MPI_LIMB);
-		p += BYTES_PER_MPI_LIMB;
-		if (lzeros > 0) {
-			mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
-			mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
-				+ lzeros;
-			*limb1 = *limb2;
-			p -= lzeros;
-			lzeros -= sizeof(alimb);
-		}
+		memcpy(p, (u8 *)&alimb + lzeros, BYTES_PER_MPI_LIMB - lzeros);
+		p += BYTES_PER_MPI_LIMB - lzeros;
+		lzeros = 0;
 	}
 	return 0;
 }

2.7.4

Currently, the nbytes local variable is calculated from the len argument as follows:

  ... mpi_read_raw_from_sgl(..., unsigned int len)
  {
    unsigned nbytes;
    ...
    if (!ents)
      nbytes = 0;
    else
      nbytes = len - lzeros;
    ...
  }

Given that nbytes is derived from len in a trivial way and that the len argument is shadowed by a local len variable in several loops, this is just confusing.

Rename the len argument to nbytes and get rid of the nbytes local variable. Do the nbytes calculation in place.

---
 lib/mpi/mpicoder.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

index d995a4c..048f0aa 100644
@@ -418,15 +418,15 @@ EXPORT_SYMBOL_GPL(mpi_write_to_sgl);
  * a new MPI and reads the content of the sgl to the MPI.
  *
  * @sgl:	scatterlist to read from
- * @len:	number of bytes to read
+ * @nbytes:	number of bytes to read
  *
  * Return:	Pointer to a new MPI or NULL on error
  */
-MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int len)
+MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
 {
 	struct scatterlist *sg;
 	int x, i, j, z, lzeros, ents;
-	unsigned int nbits, nlimbs, nbytes;
+	unsigned int nbits, nlimbs;
 	mpi_limb_t a;
 	MPI val = NULL;
 

@@ -455,7 +455,7 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int len)
 	if (!ents)
 		nbytes = 0;
 	else
-		nbytes = len - lzeros;
+		nbytes -= lzeros;
 
 	nbits = nbytes * 8;
 	if (nbits > MAX_EXTERN_MPI_BITS) {

2.7.4

At the very beginning of mpi_read_raw_from_sgl(), the leading zeros of the input scatterlist are counted:

  lzeros = 0;
  for_each_sg(sgl, sg, ents, i) {
    ...
    if (/* sg contains nonzero bytes */)
      break;

    /* sg contains nothing but zeros here */
    ents--;
    lzeros = 0;

}

Later on, the total number of trailing nonzero bytes is calculated by subtracting the number of leading zero bytes from the total number of input bytes:

  nbytes -= lzeros;

However, since lzeros gets reset to zero for each completely zero leading sg in the loop above, it doesn't include those.

Besides wasting resources by allocating a too large output buffer, this mistake propagates into the calculation of x, the number of leading zeros within the most significant output limb:

  x = BYTES_PER_MPI_LIMB - nbytes % BYTES_PER_MPI_LIMB;

What's more, the low order bytes of the output, equal in number to the extra bytes in nbytes, are left uninitialized.

Fix this by adjusting nbytes for each completely zero leading scatterlist entry.

---
 lib/mpi/mpicoder.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

index 048f0aa..4ba0f23 100644
@@ -447,16 +447,12 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
 			break;
 
 		ents--;
+		nbytes -= lzeros;
 		lzeros = 0;
 	}
 
 	sgl = sg;
-
-	if (!ents)
-		nbytes = 0;
-	else
-		nbytes -= lzeros;
-
+	nbytes -= lzeros;
 	nbits = nbytes * 8;
 	if (nbits > MAX_EXTERN_MPI_BITS) {
 		pr_info("MPI: mpi too large (%u bits)\n", nbits);

2.7.4

In mpi_read_raw_from_sgl(), unsigned nbits is calculated as follows:

  nbits = nbytes * 8;

and redundantly cleared later on if nbytes == 0:

  if (nbytes > 0)
    ...
  else
    nbits = 0;

Purge this redundant clearing for the sake of clarity.

---
 lib/mpi/mpicoder.c | 2 --
 1 file changed, 2 deletions(-)

index 4ba0f23..27703aa 100644
@@ -461,8 +461,6 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
 
 	if (nbytes > 0)
 		nbits -= count_leading_zeros(*(u8 *)(sg_virt(sgl) + lzeros));
-	else
-		nbits = 0;
 
 	nlimbs = DIV_ROUND_UP(nbytes, BYTES_PER_MPI_LIMB);
 	val = mpi_alloc(nlimbs);

2.7.4

The number of bits, nbits, is calculated in mpi_read_raw_from_sgl() as follows:

  nbits = nbytes * 8;

Afterwards, the number of leading zero bits of the first byte get subtracted:

  nbits -= count_leading_zeros(*(u8 *)(sg_virt(sgl) + lzeros));

However, count_leading_zeros() takes an unsigned long and thus, the u8 gets promoted to an unsigned long.

Thus, the above doesn't subtract the number of leading zeros in the most significant nonzero input byte from nbits, but the number of leading zeros of the most significant nonzero input byte promoted to unsigned long, i.e. BITS_PER_LONG - 8 too many.

Fix this by subtracting

count_leading_zeros(...) - (BITS_PER_LONG - 8)

from nbits only.

---
 lib/mpi/mpicoder.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

index 27703aa..24a0155 100644
@@ -460,7 +460,8 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
 	}
 
 	if (nbytes > 0)
-		nbits -= count_leading_zeros(*(u8 *)(sg_virt(sgl) + lzeros));
+		nbits -= count_leading_zeros(*(u8 *)(sg_virt(sgl) + lzeros)) -
+			(BITS_PER_LONG - 8);
 
 	nlimbs = DIV_ROUND_UP(nbytes, BYTES_PER_MPI_LIMB);
 	val = mpi_alloc(nlimbs);

2.7.4

Within the byte reading loop in mpi_read_raw_sgl(), there are two housekeeping indices used, z and x.

At all times, the index z represents the number of output bytes covered by the input SGEs for which processing has completed so far. This includes any leading zero bytes within the most significant limb.

The index x changes its meaning after the first outer loop's iteration
though: while processing the first input SGE, it represents

"number of leading zero bytes in most significant output limb" +
"current position within current SGE"

For the remaining SGEs OTOH, x corresponds just to

"current position within current SGE"

After all, it is only the sum of z and x that has any meaning for the output buffer and thus, the

"number of leading zero bytes in most significant output limb"

part can be moved away from x into z from the beginning, opening up the opportunity for cleaner code.

Before the outer loop iterating over the SGEs, don't initialize z with zero, but with the number of leading zero bytes in the most significant output limb. For the inner loop iterating over a single SGE's bytes, get rid of the buf_shift offset to x' bounds and let x run from zero to sg->length - 1.

---
 lib/mpi/mpicoder.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

index 24a0155..a9f1097 100644
@@ -477,19 +477,17 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
 
 	j = nlimbs - 1;
 	a = 0;
-	z = 0;
-	x = BYTES_PER_MPI_LIMB - nbytes % BYTES_PER_MPI_LIMB;
-	x %= BYTES_PER_MPI_LIMB;
+	z = BYTES_PER_MPI_LIMB - nbytes % BYTES_PER_MPI_LIMB;
+	z %= BYTES_PER_MPI_LIMB;
 
 	for_each_sg(sgl, sg, ents, i) {
 		const u8 *buffer = sg_virt(sg) + lzeros;
 		int len = sg->length - lzeros;
-		int buf_shift = x;
 
 		if  (sg_is_last(sg) && (len % BYTES_PER_MPI_LIMB))
 			len += BYTES_PER_MPI_LIMB - (len % BYTES_PER_MPI_LIMB);
 
-		for (; x < len + buf_shift; x++) {
+		for (x = 0; x < len; x++) {
 			a <<= 8;
 			a |= *buffer++;
 			if (((z + x + 1) % BYTES_PER_MPI_LIMB) == 0) {

@@ -498,7 +496,6 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
 			}
 		}
 		z += x;
-		x = 0;
 		lzeros = 0;
 	}
 	return val;

2.7.4

Within the copying loop in mpi_read_raw_from_sgl(), the last input SGE's byte count gets artificially extended as follows:

  if (sg_is_last(sg) && (len % BYTES_PER_MPI_LIMB))
    len += BYTES_PER_MPI_LIMB - (len % BYTES_PER_MPI_LIMB);

Within the following byte copying loop, this causes reads beyond that SGE's allocated buffer:

  BUG: KASAN: slab-out-of-bounds in mpi_read_raw_from_sgl+0x331/0x650
                                     at addr ffff8801e168d4d8
  Read of size 1 by task systemd-udevd/721
  [...]
  Call Trace:
   [<ffffffff818c4d35>] dump_stack+0xbc/0x117
   [<ffffffff818c4c79>] ? _atomic_dec_and_lock+0x169/0x169
   [<ffffffff814af5d1>] ? print_section+0x61/0xb0
   [<ffffffff814b1109>] print_trailer+0x179/0x2c0
   [<ffffffff814bc524>] object_err+0x34/0x40
   [<ffffffff814bfdc7>] kasan_report_error+0x307/0x8c0
   [<ffffffff814bf315>] ? kasan_unpoison_shadow+0x35/0x50
   [<ffffffff814bf38e>] ? kasan_kmalloc+0x5e/0x70
   [<ffffffff814c0ad1>] kasan_report+0x71/0xa0
   [<ffffffff81938171>] ? mpi_read_raw_from_sgl+0x331/0x650
   [<ffffffff814bf1a6>] __asan_load1+0x46/0x50
   [<ffffffff81938171>] mpi_read_raw_from_sgl+0x331/0x650
   [<ffffffff817f41b6>] rsa_verify+0x106/0x260
   [<ffffffff817f40b0>] ? rsa_set_pub_key+0xf0/0xf0
   [<ffffffff818edc79>] ? sg_init_table+0x29/0x50
   [<ffffffff817f4d22>] ? pkcs1pad_sg_set_buf+0xb2/0x2e0
   [<ffffffff817f5b74>] pkcs1pad_verify+0x1f4/0x2b0
   [<ffffffff81831057>] public_key_verify_signature+0x3a7/0x5e0
   [<ffffffff81830cb0>] ? public_key_describe+0x80/0x80
   [<ffffffff817830f0>] ? keyring_search_aux+0x150/0x150
   [<ffffffff818334a4>] ? x509_request_asymmetric_key+0x114/0x370
   [<ffffffff814b83f0>] ? kfree+0x220/0x370
   [<ffffffff818312c2>] public_key_verify_signature_2+0x32/0x50
   [<ffffffff81830b5c>] verify_signature+0x7c/0xb0
   [<ffffffff81835d0c>] pkcs7_validate_trust+0x42c/0x5f0
   [<ffffffff813c391a>] system_verify_data+0xca/0x170
   [<ffffffff813c3850>] ? top_trace_array+0x9b/0x9b
   [<ffffffff81510b29>] ? __vfs_read+0x279/0x3d0
   [<ffffffff8129372f>] mod_verify_sig+0x1ff/0x290
  [...]

The exact purpose of the len extension isn't clear to me, but due to its form, I suspect that it's a leftover somehow accounting for leading zero bytes within the most significant output limb.

Note however that without that len adjustement, the total number of bytes ever processed by the inner loop equals nbytes and thus, the last output limb gets written at this point. Thus the net effect of the len adjustement cited above is just to keep the inner loop running for some more iterations, namely < BYTES_PER_MPI_LIMB ones, reading some extra bytes from beyond the last SGE's buffer and discarding them afterwards.

Fix this issue by purging the extension of len beyond the last input SGE's buffer length.

---
 lib/mpi/mpicoder.c | 3 ---
 1 file changed, 3 deletions(-)

index a9f1097..747606f 100644
@@ -484,9 +484,6 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
 		const u8 *buffer = sg_virt(sg) + lzeros;
 		int len = sg->length - lzeros;
 
-		if  (sg_is_last(sg) && (len % BYTES_PER_MPI_LIMB))
-			len += BYTES_PER_MPI_LIMB - (len % BYTES_PER_MPI_LIMB);
-
 		for (x = 0; x < len; x++) {
 			a <<= 8;
 			a |= *buffer++;

2.7.4

All applied.

Email: Herbert Xu <[email protected]>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Thank you very much :)